1. Field of the Invention
The present invention relates to a small and lightweight heat exchanger suitable for use in an air conditioner for vehicles and a method for manufacturing the same.
2. Description of the Related Art
JP-A-SHO 61-153388 and JP-A-SHO 61-153389 disclose a heat exchanger in which heat exchanging tubes having a diameter of not more than 3 mm are disposed at a pitch of not more than 3 mm and the tubes are connected by meshes provided instead of fins for increasing the heat transfer area and improving the efficiency of heat exchange. In such a heat exchanger, however, as the number of heat transfer tubes increases, handling and assembly become more difficult, the number of meshes increases and the cost of the heat exchanger increases.
U.S. Pat. No. 4,235,281 discloses a heat exchanger in which a pair of thin film sheets each having recessed portions are bonded to each other to form a panel unit having tube portions and plate portions connecting adjacent tube portions. In this heat exchanger, the thin film sheets are plastic film sheets with a thickness of 0.01-0.25 mm. Generally, a metal such as aluminium or an aluminum alloy is used as the material for a heat exchanger for vehicles. If the structure of U.S. Pat. No. 4,235,281 were to be used for a heat exchanger for vehicles, the thickness of each tube must be set to at least about 0.4 mm to satisfy the requirements for pressure resistance and corrosion resistance. Consequently, because a pair of sheets are bonded to form a panel unit in the structure of U.S. Pat. No. 4,235,281, the thickness of plate portions formed by such a method becomes at least 0.8 mm. A lightweight heat exchanger cannot be achieved by such a structure.
Further, in a conventional method for merely forming a panel unit 101 having tube portions 102 with a thickness t.sub.1 and plate portions 103 with a thickness t.sub.2, as shown in FIG. 8, by extrusion, it is difficult to set the thickness t.sub.2 of plate portions 103 to a small value and the width "w" of plate portions 103 to a large value due at least in part to flowability constraints of the material being extruded. As a result, a lightweight heat exchanger cannot be achieved by such a method.